PHP - 如何更快,更快地编写IF和ELSE语句

Question

我之前写的这段代码.它检查外部服务器是否有图像,如果存在,它将回显列表中的图像.我有最多13条记录要检查.我认为阵列在这里最好但不确定它们是如何工作的.

    $a1="".$cdnurl."assets/".$pid."/1/medium.jpg";
              $a2="".$cdnurl."assets/".$pid."/2/medium.jpg";
              $a3="".$cdnurl."assets/".$pid."/3/medium.jpg";
              $a4="".$cdnurl."assets/".$pid."/4/medium.jpg";
              $a5="".$cdnurl."assets/".$pid."/5/medium.jpg";
              $a6="".$cdnurl."assets/".$pid."/6/medium.jpg";
              $a7="".$cdnurl."assets/".$pid."/7/medium.jpg";
              $a8="".$cdnurl."assets/".$pid."/8/medium.jpg";
              $a9="".$cdnurl."assets/".$pid."/9/medium.jpg";
              $a10="".$cdnurl."assets/".$pid."/10/medium.jpg";
              $a11="".$cdnurl."assets/".$pid."/11/medium.jpg";
              $a12="".$cdnurl."assets/".$pid."/12/medium.jpg";
              $a13="".$cdnurl."assets/".$pid."/13/medium.jpg";
              if(@fopen($a1,"r")){
                  echo '<li class="royalSlide"><img src="'.$a1.'" alt="" /></li>';
            }
if(@fopen($a2,"r")){echo '<li class="royalSlide"><img src="'.$a2.'" alt="" /></li>';
}
if(@fopen($a3,"r")){echo '<li class="royalSlide"><img src="'.$a3.'" alt="" /></li>';
}
if(@fopen($a4,"r")){echo '<li class="royalSlide"><img src="'.$a4.'" alt="" /></li>';
}
if(@fopen($a5,"r")){echo '<li class="royalSlide"><img src="'.$a5.'" alt="" /></li>';
}
if(@fopen($a6,"r")){echo '<li class="royalSlide"><img src="'.$a6.'" alt="" /></li>';
}
if(@fopen($a7,"r")){echo '<li class="royalSlide"><img src="'.$a7.'" alt="" /></li>';
}
if(@fopen($a8,"r")){echo '<li class="royalSlide"><img src="'.$a8.'" alt="" /></li>';
}
if(@fopen($a9,"r")){echo '<li class="royalSlide"><img src="'.$a9.'" alt="" /></li>';
}
if(@fopen($a10,"r")){echo '<li class="royalSlide"><img src="'.$a10.'" alt="" /></li>';
}
if(@fopen($a11,"r")){echo '<li class="royalSlide"><img src="'.$a11.'" alt="" /></li>';
}
if(@fopen($a12,"r")){echo '<li class="royalSlide"><img src="'.$a12.'" alt="" /></li>';
}

Answer 1

for ($i = 1; $i < 14; $i++)
{
   $elem = $cdnurl . "assets/" . $pid . "/$i/medium.jpg";
   if(is_readable($elem))
   {
      echo '<li class="royalSlide"><img src="'.$elem.'" alt="" /></li>';
   }
}